This is the best book that will get you up and running on Wireshark the fastest... only 192 pages! It even describes various protocols in many practical exercises, which is very important in security work:

Laura Chappell at the Protocol Analysis Institute has authored some great books and has a number of resources at http://www.packet-level.com/. I learned quite a bit about networking and IP in general from her Intro and Advance Network Analysis books.